1. Field of the Invention
This invention relates to silicone/acrylate vibration dampers.
2. Description of the Related Art
Damping is the dissipation of mechanical energy as heat by a material in contact with the source of that energy. It is desirable that the damping occur over a wide range of temperatures (e.g. -50.degree. C. to 200.degree. C.) and a broad frequency range from as low as about 0.1 to 5 Hertz (Hz), the frequency range over which tall buildings experience wind sway or seismic vibrations, to higher frequencies such as 1 kilohertz (KHz) experienced by computer disk drives, to even higher frequencies of 5 to 10 KHz experienced in high cycle fatigue applications.
For damping applications, it is further desirable that the damping material, sometimes referred to as viscoelastic material, have the following properties: (1) damping capabilities at high temperatures e.g., at 50.degree. C. and above, preferably 100.degree. C. and above; and (2) damping performance that is substantially independent of temperature over the useful temperature range.
In terms of measured characteristics, the composition should ideally have a high loss factor (tan .delta.) and a modulus (G', G") that is independent of temperature. In some applications, it may be desirable for the damping material to possess adhesive properties.
Until now, industry has sacrificed bond strength and broad temperature coverage for the ability to provide damping performance at elevated temperatures. Applications where damping is required over a broad temperature range include aircraft skins, airplane engine stationary inlet guide vanes, and satellite components.
U.S. Pat. No. 4,942,187 (Kawata et al.) discloses a vibration-damping rubber composition comprising: (A) 100 parts by weight hexene-1 type polymer and a Si-containing graft copolymer; (B) 5-50 parts by weight inorganic reinforcing agent; (C) 0-50 parts by weight processing oil; and (D) 0.1-5 parts by weight crosslinking agent. Although "vibration insulating" properties and resistance to permanent compression set are disclosed, the compositions show losses of from 69-85% of the 100% modulus value when heat aged at 175.degree. C. for 72 hours, and a loss factor of less than 0.5 at 50.degree. C. and 100 Hz.
European Patent Publication No. 0,390,207 (Yagi et al.) discloses a vibration proof damping material prepared from a blend of (a) amorphous fluorine-containing polymer or a crystalline vinylidene fluoride containing polymer and (b) acrylic polymer. The material is stated to provide vibration damping for noise sources and stability in shape over a wide temperature range.
U.S. Pat. No. 5,085,364 (Ishikawa et al.) discloses a pressure sensitive adhesive composition for temporarily securing surface-mount devices on circuit boards. The composition comprises a suitable resinous material having a dynamic modulus of 10.sup.4 to 10.sup.9 dynes/cm.sup.2 at 200.degree. C.-280.degree. C. at a frequency of 10 Hz. Examples of resinous materials include acrylate copolymers, silicone rubbers, silicone-acrylate copolymers, diene rubbers, vinyl ether resins, vinyl pyrrolidone-based resins, polysaccharides, amylopectin, and plant gums. Dynamic mechanical properties are reported at 260.degree. C. and a frequency of 10 Hz.
Thus a need exists for vibration damping materials having bond strength, elevated temperature damping performance and damping over a broad temperature range of about -50.degree. C. to about 200.degree. C. While silicone adhesives and acrylates are known as vibration damping materials, the present invention suprisingly discloses that compositions based on silicone/acrylate blends provide a synergistic performance and are especially useful for vibration damping. The compositions having a storage modulus, G', above the rubbery plateau, e.g., above about 0.01 megapascals (MPa) at 3 Hz and a loss tangent, tan .delta., above 0.5, preferably above 0.7 over significant portions of the temperature range of about -50.degree. C. to about 200.degree. C. at a frequency of 3 Hz.